One of my websites was recently banned from the google index, and I can't figure out why. Does anyone know if there is a way to figure that out? Or if there is a way to contact google and ask them?
First Check your robots.txt file if some one changed as disallow the google Bot...another reason is someone who copied your website as it is..and Google offers the benefit of doubt to who copied your website..to find duplicate content which used your content try copyscape.com
If you are sure the site is banned (ie not in the index) the best suggestion is to look through the google webmaster guidelines and see if you have broken any. These guidelines include buying and selling links, among other things. Then after you have got rid of any problems you can ask for 'reinclusion' in google webmasters if you tell them what you did wrong in the first place. But if you have lots of 'infractions' you won't know which one they spotted so will need to remove all of them! And if you did nothing wrong it's less obvious what you do, you could try requesting reinclusion and explaining you did nothing wrong.
search in google site:yourdomain.com No results appear there indicates that your site is banned.It means that none of your pages are indexed in google